1. Field of the Invention
The present invention relates to a displacement sensor and, more particularly, to an encoder.
2. Description of the Related Art
Conventionally, an encoder used as a displacement sensor has been known. The encoder outputs a plurality of periodic signals different in phase from each other. The output signals are input to a counter and counted, detecting the displacement of a target object or the like based on the count processing result. To enable the count processing by the counter, the encoder outputs signals having undergone predetermined processing by a processing circuit which performs, for example, conversion processing into a digital signal and processing for increasing the displacement detection resolution.
Such an encoder 1000 can measure the traveling direction, position, displacement, displacement speed, and the like of a target object. The encoder 1000 is generally configured as shown in FIG. 6. More specifically, the encoder 1000 comprises an original signal generating means 1001 and original signal processing means 1002.
The original signal generating means 1001 includes a light emitting element 1101, a rotating plate 1102, a stationary plate 1103, a light receiving element 1104, current-to-voltage converters 1105, and differential circuits 1106. The rotating plate 1102 is fixed to a motor shaft and has optical slits. The stationary plate 1103 is fixed to a motor bracket and has at least four segments of slits identical to those of the rotating plate 1102. The light receiving element 1104 has light receiving portions corresponding to at least the segments of the stationary plate 1103. Each current-to-voltage converter 1105 converts an output current from the light receiving element 1104 into a voltage. Each differential circuit 1106 calculates the difference between signals converted into voltages by the current-to-voltage converters 1105.
In the encoder 1000 having this arrangement, light emitted by the light emitting element 1101 passes through the slits of the rotating plate 1102 and stationary plate 1103 and enters the light receiving element 1104. The light receiving element 1104 outputs a current corresponding to the incident light.
The slits of the rotating plate 1102 and the slits of the respective segments of the stationary plate 1103 are formed so that, as the rotating plate 1102 rotates, the current is output as pseudo sine-wave signals a and b serving as signals of two phases with a phase difference of about 90°, and signals ā and b opposite in phase to the signals a and b.
The current-to-voltage converters 1105 convert the pseudo sine-wave signals a and b and the opposite-phase signals ā and b output from the light receiving element 1104 into voltage signals using a reference voltage as the reference.
To remove the DC component of an optical signal, the differential circuits 1106 calculate CA=a−ā, CB=b−b, −CA=ā−a using a reference voltage as the reference, outputting pseudo sine-wave signals of three phases.
More specifically, CA and CB are pseudo sine-wave signals with a phase difference of about 90°, and CA and −CA are pseudo sine-wave signals with a phase difference of about 180°.
The original signal processing means 1002 performs multiplication processing to divide the pseudo sine-wave signals of three phases output from the original signal generating means 1001 by a predetermined division number. Encoder square-wave signals obtained by the multiplication processing by the original signal processing means 1002 are signals countable by the counter having a predetermined resolution.
Details of the original signal processing means 1002 are as follows. The original signal processing means 1002 includes a sine-wave signal generating means 1201, impedance matching means 1202, comparing means 1203, and signal multiplying means 1204.
The sine-wave signal generating means 1201 resistance-divides the pseudo sine-wave signals CA, CB, and −CA output from the original signal generating means 1001, generating a plurality of pseudo sine-wave signals different in phase.
The impedance matching means 1202 executes output impedance matching of a signal line, and phase adjustment by CR coupling with the input capacitance of the subsequent comparing means 1203.
The comparing means 1203 binarizes a pseudo sine-wave signal output from the sine-wave signal generating means 1201 using the reference voltage, generating a digital signal.
The signal multiplying means 1204 performs logical processing for the digital signal output from the comparing means 1203, generating an encoder square-wave signal.
Owing to variations of the current-to-voltage converters 1105 and differential circuits 1106 and the like, different offset voltages exist between the center voltages of the pseudo sine-wave signals CA, CB, and −CA of three phases output from the original signal generating means 1001, and the reference voltage.
The offset voltage generates a phase error in processes by the sine-wave signal generating means 1201 and comparing means 1203 in the original signal processing means 1002, and impairs even the linearity of an encoder square-wave signal output from the original signal processing means 1002. When the amplitudes of the pseudo sine-wave signals CA, CB, and −CA are small, the encoder square-wave signal itself may not be generated.